1. Field of the Invention
The present application relates in general to hydraulic machines, and in particular to a yokeless pump/motor with a sliding valve plate.
2. Description of the Related Art
Pump/motors having sliding valve plates are well known in the industry. An advantage that such motors have over pump/motors employing a yoke and trunnion for displacement control is fewer moving parts. However, for reasons that will be explained below, sliding valve plate pump/motors are generally limited as to the maximum stroke angle possible. Inasmuch as maximum available efficiency and energy transfer are directly related to maximum stroke angle, a long-sought goal has been the development of sliding valve plate pump/motors capable of displacement angles greater than around 20 degrees.
Referring to FIG. 1, the back plate 100 of a known pump/motor is shown. The sliding surface 102 may be seen, whereon a valve plate is configured to ride. The lateral position of the valve plate 108, as shown in FIG. 2, is controlled by the rocking pin 106. Fluid feed apertures 104 provide high and low pressure fluid to the valve plate 108.
The back side 108b of the valve plate 108 may be seen in FIG. 2. The valve plate 108 includes fluid feed channels 112 configured to receive fluid from the fluid feed apertures 104 of the back plate 100, and to transmit that fluid to the piston barrel of the pump, via the kidney slots, or valve slots 116, visible through the fluid feed channels 112, and more easily visible in FIG. 3. Sealing lands 110 provide a seal between the sliding surface 102 of the back plate and the valve plate 108.
FIG. 3 shows the top surface 108a of valve plate 108. The top surface 108a includes the valve slots 116, the annular sealing land 118, and the barrel pin 103. A cylinder barrel is configured to sit on the top surface 108a of the valve plate 108 and engage the barrel pin 103. When operating in motor mode, cylinder ports in a bottom surface of the barrel receive high-pressure fluid from one of the valve slots 116 and, as the barrel rotates, discharge the fluid into the opposite side valve slot 116, in a known manner.
The displacement of the pump/motor, and hence the degree of energy transfer, is determined by the angle of an axis of the barrel relative to an axis of a thrust plate and output shaft of the pump/motor. This is sometimes referred to as the stroke angle of the machine. The rocking pin 106, shown in FIG. 1, is configured to engage the rocking bore 114 of FIG. 2 for the purpose of adjusting the angle of the barrel.
By comparing the bottom surface 108b of the valve plate 108 with the back plate 100, it may be seen that the travel of the valve plate 108 over the back plate 100 is limited by the length of the fluid feed channels 112, and the length of the sliding surface 102. It will be understood that in order for the pump/motor to function properly, the sliding surface 102 must be sufficiently broad such that when the valve plate is at either extreme end of its travel, the entire length of each of the sealing lands 110 is in contact with the sliding surface 102. Additionally, when the valve plate 108 is at either extreme, the fluid feed apertures 104 must have access to the fluid feed channels 112. Thus, it would seem a simple matter, in order to produce a pump/motor capable of greater displacement angles, to manufacture a valve plate having longer fluid feed channels 112, and correspondingly broader sliding surfaces 102. However, significant design problems arise when such modifications are attempted.
Reference is made to FIGS. 4 and 5 to facilitate an explanation of the problems associated with changing the dimensions of the fluid feed channel 112.
Where the value n is used in the figures and descriptive text to indicate an undefined quantity, it will be understood that any number of the indicated feature may be appropriate. For example, in the case of drive cylinders and pistons, as described below, an odd number, such as seven or nine, is generally employed, though machines utilizing other quantities are also known.
FIGS. 4 and 5 show diagrammatical cross-sections of a sliding valve plate pump/motor 133 of a type similar to that illustrated in FIGS. 1-3. More particularly, FIG. 4 shows a cross-section taken in a plane X, indicated in FIG. 5 at lines 4-4, while FIG. 5 is taken in a plane Y. FIGS. 4 and 5 are diagrammatical in nature, and do not represent a functional machine. In particular, the cylindrical barrel 107 and semicircular kidney port 117 of FIG. 5 are depicted as being flat or planar for the purpose of describing forces acting on the various components of the pump/motor.
The pump/motor 133 of FIGS. 4 and 5 includes a back plate 101, a valve plate 109, and a barrel 107. Pistons 111a-111n are positioned within respective cylinders 115a-115n. Pressurized fluid is provided to the pump/motor 133 via fluid feed passage 121 and fluid feed aperture 105. The pressurized fluid passes into the valve plate 109 via the fluid feed channel 113, and from the valve plate 109 to the barrel 107 via the valve slot 117. The fluid enters each of the cylinders 115 via cylinder ports 123a-123n. 
Pascal's law teaches that a pressurized fluid in an enclosed space exerts equal pressure on all surfaces of that space. Accordingly, with reference to FIG. 4, fluid entering cylinder 115b via cylinder port 123b will exert equal pressure on all surfaces within the cylinder 115b. Assuming that the pump/motor 133 is functioning as a motor, and that the fluid entering the fluid feed passage 121 is at a drive pressure, the pressure of the fluid will drive the piston 111b in an upward direction. Since force acting on the piston 111b in an upward direction is not transmitted to the barrel 107, there is substantially no upward force exerted on the barrel 107, by fluid inside the cylinder 115b. However, the pressurized fluid is also acting on the cylinder's shoulders 119 in a downward direction, pushing the barrel downward onto the valve plate 109, and the valve plate 109 downward onto the back plate 101. Inasmuch as FIG. 4 shows no surfaces of the valve plate 109 on which the fluid is acting, there is a net downward force from the barrel 107, through the valve plate 109, to the back plate 101, with respect to the surfaces shown in FIG. 4. This is sometimes referred to as the clamping force, and, in most known sliding valve plate systems, is the major force holding the barrel 107 and valve plate 109 against the back plate 101.
Referring now to FIG. 5, it may be seen that, in the Y plane, there are several surfaces upon which pressurized fluid may act to generate upward force. For example, the barrel 107 has a surface 125 that is in contact with pressurized fluid, which affects the net clamping force of the cylinder barrel 107. Additionally, valve plate 109 has interior surfaces 131 upon which pressurized fluid will exert upward pressure. Finally, there is a pressure gradient across the sealing lands 110 (see FIG. 2) that imposes a net upward force on the valve plate 109.
It will be understood that, in order for the pump/motor 133 to function properly, the total downward force acting on the valve plate 109 must exceed the total upward force, to prevent the valve plate 109 from lifting from its position. The sum of these forces can be referred to as the net lifting force. The net lifting force F acting on the valve plate 109 of the pump/motor 133 may be approximated as follows:
                    F        =                                            (                              C                +                G                            )                        ⁢                          in              2                        ×                                          lb                .                                            in                2                                              -          B                                    Formula        ⁢                                  ⁢        1            
Where C is equal to the total area of the fluid feed channel 113 minus the total area of the valve slot 117, G is equal to half the total area of the sealing lands 110, B represents the net clamping force of the cylinder barrel 107, and the pounds per square inch represents the fluid pressure in psi.
As long as the resulting value of F is a negative value, the pump/motor 133 will function properly. However, if the resulting figure is a positive value, the barrel 107 and the valve plate 109 will not remain properly seated, and pressurized fluid will escape from the system, preventing the pump/motor 133 from functioning. In simple terms, the net clamping force of the barrel 107 must be greater than the sum of forces acting on the sealing lands 110 and the horizontal component of the surfaces 131 of the valve plate.
Returning now to the question of lengthening the fluid feed channel in order to improve the maximum displacement capability of the pump/motor 133, it may be seen that, as the dimension CY, representing the length of the fluid feed channel 113, increases, so too will the surface area 131 of the valve plate 109. As the surface area 131 increases, the upward forces acting on that surface area will very quickly overcome the downward forces acting on surface areas 119 to cause the valve plate 109 to separate from the back plate 101. A common response to this problem has been to increase the surface area of the shoulders 119 of the cylinders 115a-115n. To do this, the cylinder ports 123 are narrowed in the dimension indicated at Px of FIG. 4, thus broadening the shoulders 119. However, when the dimension Px is reduced, the width of the valve slot 117, the fluid feed channel 113, and the fluid feed aperture 105, indicated in FIG. 4 as dimensions Sx, Cx, and Bx, respectively, must each be reduced in turn. This results in narrowing the fluid passages, especially the fluid passing through the fluid feed aperture 105, and entering the cylinder ports 123. As a result, the rate of fluid transfer into the cylinders 115a-115n is reduced, or choked, reducing the efficiency with which the motor transfers energy. Thus, in order to increase the maximum displacement angle of the pump/motor 133, efficiency is sacrificed.